1. Field of the Invention
The present invention relates to an inkjet ink having a reactive colorant that is less susceptible to hydrolysis, thereby having an improved shelf life. The reactive colorant reacts with an activating agent to fix the reactive colorant to a textile substrate.
2. State of the Art
Images are typically printed on textiles using various colorants, such as dyes or pigments. For example, when printing on textiles using an inkjet printer, the inkjet inks use acid dyes, disperse dyes, reactive dyes, or pigments as the colorants. Reactive dyes crosslink or bind to the fibers of the textile, thereby providing the desired images on the fibers. The reactive dye has a chromophore and a reactive group, such as a halotriazine or halodiazine group, which fixes the dye to the textile. In contrast, pigments are dispersed with a large amount of polymer resin, which keeps the pigment dispersed and acts as a binder to the textile. The colorants generally used to print on textiles are shipped in a dry form and reconstituted into a paste for use. The paste-like colorant is then applied to the textile to form the desired image. For example, in screen printing, the colorant is pressed through a screen to produce the desired image on the textile. Heat is then applied to set the colorant to prevent it from fading or rinsing out during washing. While this, and similar, processes of printing on textiles have been used for hundreds of years, these processes are not optimal because they require multiple steps and are time consuming.
The reliability, high print quality, and low cost of inkjet printing have provided an attractive alternative for improving the speed of printing on textiles. Therefore, inkjet inks compatible with textiles have been, and continue to be, developed. Similar to the colorants used in non-inkjet printing applications, inkjet inks use a dye or pigment as the colorant. However, the colorant is formulated in a liquid ink vehicle rather than in a paste. The colorant used in an inkjet ink also has a reactive group that reacts with the textile fibers to fix the colorant to the textile. The reactivity of the colorant depends on the reactivity of the reactive group with the textile fibers. For example, the reactivity of reactive dyes, which have a reactive group and a chromophore (or chromogen), is generally divided into three categories. Low reactivity dyes require a high temperature, generally greater than 80° C., to react with the textile fibers. Medium reactivity dyes react at 30-60° C. and high reactivity dyes react at room temperature.
One common problem with reactive dyes, especially high and medium reactivity dyes, is that the reactive group is susceptible to hydrolysis. Since inkjets inks are shipped in aqueous solutions in an inkjet pen, the reactive group will hydrolyze if the inkjet ink is not used within a certain period of time (referred to as the shelf life of the inkjet ink) or if the inkjet ink is stored at a high temperature. If the reactive group is hydrolyzed, the colorant will not effectively fix to the substrate and the color of the printed image will not be optimal.
Inkjet inks that are currently available to print on textiles are not optimal because the inkjet inks do not adhere to the textile and run, smear, or wash away with repeated use or laundering. Furthermore, pigment-based inkjet inks are generally problematic for use in inkjet printing because they have high concentrations of binder (up to 20-30%) in the inkjet ink. In addition, many of these inkjet inks do not have a sufficiently long shelf life. While inkjet inks that use pigments, acid dyes, or disperse dyes have a shelf life of multiple years, inkjet inks that use reactive dyes as the colorant have a shelf life of approximately twelve months. Therefore, inkjet inks that use reactive dyes are only useful for a time period up to about twelve months. The twelve-month time period is measured from the date of manufacture of the ink and not from the date at which the ink reaches the customer. This short shelf life imposes severe restraints on ink and pen distribution because the inkjet ink commonly degrades in the amount of time that it takes to distribute the ink to the final end customers.